Question:If the band I am recording has a PA system
that is putting out a mono signal, why would I want to record in stereo
(or Binaural)? Isn't that pointless?

Answer:

Some quick definitions:

Mono=one
channel of audio signal

Stereo=
two different channels of audio signal, recorded with two microphones
spaced apart (or with a single microphone with two elements)

Even though many
bands will put out a mono signal through their PA system, it is almost
always better to record in stereo (or Binaural). Here are the reasons:

In
many cases, especially in smaller venues, some of the sound you hear
is coming right off the stage and is essentially "mixed in"
with the sound coming out of the PA. In this case, you are indeed
hearing the benefit of multiple sound sources coming from different
directions. Recording in mono would not capture the true essence of
the sound you heard at the time of the recording.

Even when all of
the sound is coming from the PA system, the acoustics of the venue you
are recording in greatly affects the way sounds hit your ears. By the
time the sound waves get to you, they are indeed in "stereo"
and will greatly benefit from a stereo or binaural recording. Note:
the exception to this would be when you are recording from a position
very close to the PA speakers ("stacks). In this case, most of
what your microphones will pick up is direct sound from the PA, and
very little reflected sound from the room. In this case, you would still
hear a small difference with a stereo recording, especially when using
omnidirectional microphones, but to a lesser extent than a situation
when you are a little further away from the speakers.